1. Field of the Invention
This invention is directed to an accessory device for accurately controlling the flow rate of drugs from drug delivery devices, implanted or external, that depend on a fluid restriction to limit the flow rate from a pressurized drug storage chamber. One device of this type is the implantable infusion pump illustrated and described in U.S. Pat. No. 3,731,681, the disclosure of which is incorporated herein by reference.
2. The Prior Art
U.S. Pat. No. 3,731,681 illustrates and describes an implantable infusion pump, or so-called "artificial gland", which uses a liquid/vapor equilibrium to maintain a constant pressure on a drug, such as insulin, flowing through a capillary tube in order to maintain a constant flow rate. This technique of flow control is sensitive to temperature and atmospheric pressure. Because the temperature of the human or animal body remains relatively constant, the vapor pressure also stays constant. Where the patient remains in a local region, the air pressure is a minor variable. However, there are conditions under which both temperature and pressure can change a significant amount. If the patient has a fever, or works in a cold environment, the temperature of the implanted gland can change several degrees. The internal pressure change is about 0.5 psi per degree F. A 25 percent increase in pressure and drug flow rate can result from a fever of 102.5.degree. F. This change is more than can be tolerated when a critical drug is being administered.
A more serious situation results from the reduced air pressure in airplane cabins when the patient is traveling. The standard airplane pressure is maintained at a level corresponding to an altitude of about 5,000 feet above sea level. With a gland using an 8.2 psi internal pressure, this would increase the differential pressure by 26 percent over the sea level setting. Although the drug dosage can be adjusted by changing the concentration of drug in the gland, this is a serious inconvenience and hardship for patients who must travel by air frequently.